Field of the Invention
The invention relates in general to receiving signals, and more particularly, to receiving multiple carrier signals.
Description of the Related Art
The finiteness of radio spectrum is an inevitable issue in the modern life. Transmission via multiple carrier signals yields a better spectrum utilization rate. For example, in an orthogonal frequency division multiple access (OFDMA) signal, one band may include multiple subcarriers. There are more and more standards based on OFDMA, including Digital Video Broadcasting-Terrestrial (DVB), Digital Video Broadcasting-Second Generation Terrestrial (DVB-T2), and Integrated Services Digital Broadcasting (ISDB).
Moreover, as diversified radio waves are cramped in a narrow spectrum, radio waves having similar frequencies inevitably interfere nearby frequencies. Such occurrence is referred to as an adjacent channel interference (ACI). From a timeline aspect, some adjacent channel interferences are instantaneous, while others may last for a quite long period. At least for OFDMA signals, these adjacent channel interferences last for a period of at least one symbol. From a frequency aspect, some adjacent channel interferences cover an entire band, while others may cover only a part of subcarriers of the band. In other words, by segmenting a band at a center frequency of the band, adjacent channel interferences may affect the high-frequency part or the low-frequency part.
For instantaneous adjacent channel interferences that cover an entire band, a receiver may perform a special process on the interfered symbols. However, for adjacent channel interferences that continuously cover a part of a band, a receiver needs to first know which subcarriers are interfered before being able to perform a special process on that part suffering from the adjacent channel interferences.
FIG. 1A shows a schematic diagram of a typical adjacent channel interference, where the horizontal axis represents the frequency. A band 100 is a frequency band occupied by a signal based on multiple carriers. The signal includes multiple subcarriers, denoted from 110A to 110I. In FIG. 1A, nine arrows pointing upwards indicate center frequencies of these subcarriers, with the center frequency of the subcarrier 110E also being a center frequency of the band 100.
In FIG. 1A, the vertical axis represents the signal strength. A surge of the signal strength resulted by an adjacent channel interference 120 is present near the center frequencies of the subcarriers 110H and 110I. The adjacent channel interference 120 at least affects the subcarriers 110H and 110I. Therefore, there is a need for a receiver capable of determining which subcarriers suffer from adjacent channel interferences in order to perform a special process on the interfered subcarriers.